A laser gyro is known, in particular from document FR 2 360 885 (U.S. Pat. No. 4,152,071), that comprises three mirrors placed to implement a closed-loop light path for two laser beams propagating in opposite directions and modulated at identical frequencies. It is known that when the gyro is at rest, the two beams follow paths of the same length and therefore remain at frequencies that are identical, whereas when the gyro is caused to rotate, the two beams follow paths that are unequal and therefore oscillate at different frequencies, with the frequency difference being representative of the angular speed of the gyro. This frequency difference is measured by means of an interferometer serving to measure the optical phase between the two beams.
The operation of a laser gyro is satisfactory for speeds of rotation greater than a critical value that depends on the quality with which the gyro is made and in particular on the quality with which the mirrors in the gyro are made. Typically, the critical value for the speed of rotation of the gyro is of the order of one hundred degrees per hour. For speeds of rotation lying in a zone known as the “dead band” for which the value of the speed of rotation is less than the critical value, it has been found that coupling exists between the beams such that the real speed of the gyro can no longer be measured by taking the difference between the oscillation frequencies of the laser beams.
In order to enable speed of rotation to be measured in the dead band, the above-mentioned document discloses subjecting the real rotary motion of the gyro to superposed noisy sinusoidal reciprocating drive motion referred to as dither movement, at a speed that is substantially greater than the critical value for the real speed of rotation of the gyro. The dead zone of the gyro is then replaced by a series of dead zone crossings each time the dither movement reverses, and the performance of the gyro is then limited by a random walk movement which is the integral of the gyro measurement errors occurring on each crossing of the dead band. These errors are, to a large extent, due to energy being exchanged between the two laser beams giving rise to variation in the two laser intensities as a function of optical phase. This variation is known as “winking”. In order to obtain high performance from the gyro, it is therefore necessary to reduce winking.
In order to reduce winking, the above-mentioned document discloses actuating two of the mirrors of the gyro under push-pull control, causing the distance between the mirrors to vary, while keeping the total optical path length of the laser beams constant. Push-pull control makes it possible to vary phase between back-scattering from different locations so as to minimize overall back-scatter. Back-scatter can be measured via its consequences on the lased intensity of one of the laser beams by observing the winking of the lased intensity signal.
In order to ensure that push-pull control is as appropriate as possible, it is preferable for the amplitude in the oscillations of the lased intensity to be measured in the dead band since energy exchanges therein give rise to the largest amount of gyro error. To this end, the above-mentioned document starts from the assumption that the amplitude of the lased intensity increases on crossing the dead band, and it therefore proposes measuring the maximum amplitude of the lased intensity signal.
Nevertheless, it has been found that that initial assumption is not always true, with the amplitude of the lased intensity outside the dead band being capable, under certain conditions, of taking values that are greater than the amplitude of the lased intensity in the dead band. The method of the above-mentioned document then leads to errors in measuring winking and consequently to errors in implementing push-pull control.